API inconsistency between `CreateVirtual` and `Pay`

[geoknee]

• CreateVirtual accepts a full outcome.Exit type (a rich type that supports multiple assets, metadata and so on)
• Pay only accepts a single uint64.
• Right now, go-nitro only supports the native token.

Suggested fix: use a *big.Int in all API methods until such time that full outcome.Exits are supported in go-nitro.